Pat the chicken thighs completely dry with paper towels — press firmly on all sides. Dry skin is the key to crispy skin, so don't rush this step.

In a small bowl, combine the salt, pepper, smoked paprika, and garlic powder. Season the chicken thighs all over with the spice mixture, coating both the skin side and the underside.

Heat a 12-inch cast iron skillet or heavy oven-safe skillet over medium-high heat for 2 minutes until very hot. Add the oil and swirl to coat.

Place the chicken thighs skin-side down in the skillet. Do not move them. Press down gently with a spatula for the first 30 seconds to ensure full skin contact with the pan. Cook undisturbed for 12 to 14 minutes, until the skin is deep golden brown and releases easily from the pan.

Flip the chicken thighs skin-side up. Add the smashed garlic cloves and thyme sprigs to the pan around the chicken. Cook for another 10 to 12 minutes, until the internal temperature reaches 165°F at the thickest part.

Transfer the chicken to a plate and loosely tent with foil to rest. Leave the garlic and thyme in the pan.

Pour off all but about 1 tablespoon of the drippings from the skillet. Return the skillet to medium heat and add the chicken broth, scraping up any browned bits from the bottom of the pan with a wooden spoon.

Whisk in the Dijon mustard. Let the broth simmer for 3 to 4 minutes until it reduces by about one-third.

Remove the pan from heat and discard the thyme sprigs. Add the cold butter pieces and swirl the pan or whisk constantly until the butter is fully melted and the sauce looks glossy and slightly thickened.

Taste the sauce and adjust salt as needed. Return the chicken thighs to the pan skin-side up, spoon the sauce over them, and garnish with chopped parsley. Serve immediately directly from the skillet.